Considering the interest
in processes related to hydrogen storage such as CO2 hydrogenation
and formic acid (FA) decomposition, we have synthesized a set of Ir,
Rh, or Ru complexes to be tested as versatile precatalysts in these
reactions. In relation with the formation of H2 from FA,
the possible applicability of these complexes in the transfer hydrogenation
(TH) of challenging substrates as quinoline derivatives using FA/formate
as hydrogen donor has also been addressed. Bearing in mind the importance
of secondary coordination sphere interactions, N,N′ ligands
containing NH2 groups, coordinated or not to the metal
center, were used. The general formula of the new complexes are [(p-cymene)RuCl(N,N′)]X, X = Cl–,
BF4
– and [Cp*MCl(N,N′)]Cl, M =
Rh, Ir, where the N,N′ ligands are 8-aminoquinoline (HL1),
6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline
(L3). Some complexes are not active or catalyze only one process.
However, the complexes [Cp*MCl(HL1)]Cl with M = Rh, Ir are versatile
catalysts that are active in hydrogenation of quinolines, FA decomposition,
and also in CO2 hydrogenation with the iridium derivative
being more active and robust. The CO2 hydrogenation takes
place in mild conditions using only 5 bar of pressure of each gas
(CO2 and H2). The behavior of some precatalysts
in D2O and after the addition of 9 equiv of HCO2Na (pseudocatalytic conditions) has been studied in detail and mechanisms
for the FA decomposition and the hydrogenation of CO2 have
been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the
amido species with the deprotonated ligand are observed. In the case
of ruthenium, the formate complex is also detected. For the iridium
derivative, both the amido intermediate and the hydrido species have
been observed. This hydrido complex undergoes a process of umpolung
D+↔ Ir–D. All in all, the results of this
work reflect the active role of −NH2 in the transfer
of H+.